Dear Old Phone,
You don't have to use an AT&T phone with your service. Your carrier won't tell you that because they want you to buy their phones. But if you use a SIM card you can use pretty much any unlocked GSM phone. Just make sure it's unlocked. If it's not unlocked you can have someone unlock it but it costs $20+ and sometimes you have to send your phone off somewhere.
Just google "unlocked GSM phone" and you'll find a few stores selling unlocked phones.
If you have a question about a specific phone check with your carrier.
I'm with T-Mobile and I'm quite pleased with my new Cingular phone.
- no one in particular, who loves his Sony Ericsson S700i